Zebra Mussels

Zebra mussels are the scourge of the Great Lakes. They’re also one of the animal kingdom’s best scientific study models and a terrific educational tool for everything from biology to history, according to a Wayne State University specialist on the bantam bivalves. Perhaps most importantly, offered physiologist Jeffrey Ram, PhD, the zebra mussel may be able to provide clues to a cellular process that is fundamental to sexual reproduction.

Dr. Jeffrey Ram holds up a small clump of zebra mussels. One zebra mussel is less than an inch, but they cling together, sometimes forming a blanket at the bottom of lakes and other waterways. Though the zebra mussels have caused a number of ecological problems, the WSU physiology professor believes they may provide clues to a cellular process that is fundamental to sexual reproduction in all living things.

Before zebra mussels made their appearance a little more than a decade ago, the Great Lakes held an assortment of native freshwater clams. The lake water was somewhat murky, which prevented the sun from shining too deeply into the water, and so curtailed the growth of many of the aquatic plants, or "seaweed." The fisheries were making a noticeable comeback from the decades when municipalities and industries dumped seemingly anything and everything into the lakes.

Then the invasion started. Scientists believe that in about 1985, a freighter in Europe took on ballast water, giving the empty vessel the added weight necessary for a stable ocean passage to America. The ballast water also contained some zebra mussels, which have been common throughout Europe for more than a century. The freighter then traveled to southeastern Michigan and discharged its ballast water, along with the foreign stowaways, into Lake St. Clair, which connects by rivers to Lakes Huron and Erie.

The mussels found an ideal environment. Their population exploded. "Individually, they’re not real impressive to look at," Dr. Ram said, indicating the mussel’s size at about two centimeters, or less than an inch, in length. "They’re only impressive when you find them at the bottom of Lake Erie, and as far as you can see, you’ve got a layer of zebra mussels this thick," he described, holding up a three-inch span between his thumb and index finger. Their reproductive success is due in large part to the female’s ability to lay one million eggs a year.

At the same time, the invasion was recurring elsewhere as the mussels worked their way into lakes, rivers and streams near and far, often by clinging to the hulls of recreational boats. Now, the mussels are firmly established not only in the five Great Lakes, but in the Hudson and Mississippi river basins. They are also becoming more numerous in many inland waterways throughout the eastern United States.

The problems presented by the scenario are many. The zebra mussels are great filter feeders: They draw in water, filter out the tiny bits of floating food, like algae, and expel the cleared water. As their numbers increased, the lakes became clearer. The sun penetrated to new depths, and the rooted aquatic plants found in the shallower waters enjoyed their own growth spurts, which helped to generate large, dense mats of vegetation along shorelines.

The native clams, finding less and less food in the clearer water and themselves covered by heavy loads of zebra mussels, began to die off. A study of western Lake Erie five years ago found no native clams--only a nearly unending blanket of zebra mussels. The quickly changing environment appeared to be taking its toll on fish populations as well, although other causes may also share the blame.

Able to adhere to almost any solid surface, the zebra mussels began to accumulate to the point that they would sink buoys and clog water intakes at power and water plants. The latter became an expensive problem. The utility companies spent years and millions of dollars retrofitting their equipment to battle the infiltration of the clinging invertebrate animals.

Just a few miles from campus on Belle Isle, Dr. Ram uses a pole and net to collect zebra mussels that adhere to solid surfaces like this retaining wall.

Despite their reputation as the latest and greatest menace in US and Canadian waters, Dr. Ram has found that they have their positive attributes. For one thing, said the WSU physiology professor, the reproductive biology of the zebra mussel is a good model system to study meiosis, the basic process that yields the eggs and sperm necessary for sexual reproduction.

"The oocytes, the mature eggs from this animal, have a very clear cytoplasm so it’s easy to see the internal structures." The eggs are a fairly good size -- a little less than 1/10 mm in diameter -- and have little yolk, both of which permit rather easy viewing.

"You can see certain cellular processes take place in those cells quite readily, and the particular cellular process that we’re looking at is meiosis." An adult germ cell has two sets of chromosomes. During meiosis, the two sets duplicate. In females the chromosomes separate and cell divisions occur, so that an egg contains only one set of chromosomes from the original germ cell. Depending on the species, the process can pause part-way through and may even be completed after fertilization.

Dr. Ram and his students verified that early in the process, meiosis in zebra mussels pauses at its prophase stage, when the eggs have an easily observed germinal vesicle. The germinal vesicle is simply the nucleus of the eggs.

For fertilization to occur, the animal must release the eggs into the water and a chemical called serotonin must trigger meiosis to recommence, Dr. Ram said. Serotonin is a neurotransmitter, a chemical that relays nerve impulses, that is found in both zebra mussels and in humans. As meiosis continues, the germinal vesicle breaks down, which is essential for fertilization, and the process of meiosis in the zebra mussel is finally completed after fertilization.

"This same process of reinitiating meiosis prior to fertilization also occurs in mammals. Mammals don’t use serotonin to reinitiate the process--they use other chemicals--but we think that it’s very likely that the cellular mediators of germinal vesicle breakdown will turn out to be similar or related," Dr. Ram said. "The zebra mussel probably has multiple serotonin receptors of which one type is involved in mediating spawning. Other types may mediate muscle contraction." They hope to determine the structure of the serotonin receptors that mediate spawning and to understand the cellular mediators between the membrane receptors and the breakdown of the germinal vesicles.

This information probably won’t provide any weapon against zebra mussels, Dr. Ram said, but it may well provide valuable insight into the fundamental process of sexual reproduction in all living things.

Beyond the research, Dr. Ram said, the lab serves as a public educational resource through a specially-designed site on the World Wide Web (http://www.science.wayne.edu/~jram/zmussel.htm), a site he developed three years ago with his middle-school aged daughter, Natalie. "Once I had set up the site, and then asked a few people to link their pages to mine, I was sort of astounded. Right from the beginning, I was getting 10 or 15 hits a day. For the papers that I publish, maybe I’ll get 50 or 100 reprint requests total. I got that many hits on the Web site in the first week," he recalled.

As an educational tool, he said, the mussel can help teach about history, economics, ecology, biology and physiology.

Dr. Ram has found that the muscles of zebra mussels are very easy to study. He and his research group are interested in muscle physiology and pharmacology, because it might help produce a pesticide in the fight against the zebra mussels.

"When you look at the zebra mussel and try to understand why it’s a pest and a threat to North America, along with how it’s affecting the environment, you have to understand history--what was its relationship to the canal-building era in Europe that caused it to spread there, and to the Industrial Revolution--to understand why Europe isn’t having the same problems we’re having. You have to think about economics," he said.

He explained that zebra mussels invaded Europe before or early enough in the Industrial Revolution so that even the earliest power plant designers had mussel-prevention tactics in mind. "But not so with North America," Dr. Ram said. "We’ve had to do retro-fitting of power plants. Now, most of the large capital expenditures have been done in the areas that zebra mussels have infested so rapidly, and we have just increased maintenance costs."

In addition, he commented, "You have to think about ecology. By filtering out algae, how is the zebra mussel going to impact other organisms? And then there’s just the basic biological and physiological question: How does an animal like the zebra mussel manage to stay alive and function?"

Since zebra mussels are here, and probably here to stay, Dr. Ram said, he might as well make the best of a bad situation. Actually, he reported, the situation has improved. He believes the densities of zebra mussels in the Great Lakes have peaked and may even be down somewhat. "But here we’re talking about -- in some areas of the Great Lakes -- the density of the zebra mussels going from zero (before the invasion) up to 500,000 per square meter (at their peak), and now settling back down to 100,000 per square meter. It’s still a big problem."

He added, "They’re here to stay. We’re not going to get rid of them. We may be able to come up with more effective ways to kill them at sites where we want to control them, such as power plants, but we’re not going to get rid of them in the Great Lakes."